Bone Growth

Question 1

Describe the structure of bone: composition

Answer 1

composition of bone:
Matrix
35-40% organic :
28% collagen (mainly Type 1 for tensile strength)
5%
- Proteoglycans/ glycoproteins (compressive strength and calcium binding) aka hold the water
- bone promoting growth factors/ cytokines

60% inorganic :
95% Calcium hydroxyapatite (Ca10(PO4)6(OH)2)
Approx. 5% water (dry tissue)
2% cells

the collagen triple helix strands provide flexibility whereas inbedded organic hydroxyapatite crystals provide rigidity

Question 2

Describe intramembranous ossification, how is it different to endochondral and which form of ossification do most bones of the body arise from? make reference to bone remodelling

Answer 2

this happens in the flat bones of skull, mandible + part of clavicle

mesenchymal cells condense + directly give rise to bone; direct bone formation whereas endochondral goes through cartilaginous stage first (so most bones develop through endochondral ossification)

so process; mesenchymal cells condense and produce a bone matrix, these cells differentiate into osteoblasts then change into osteocytes. Then blood vessels grow into osteoblast area infiltrating it (initially the bone formed is in a woven formation so no clear lamellae/osteon structure its not neatly aligned yet) gradually the woven bone is broken down and replaced by lamella bone (this replacement is called bone remodelling). Connective tissue surrounding the bone differentiates into periosteum.

Question 3

Describe endochondral ossification

Answer 3

majority of bones formed this way

mesenchymal stem cells go through cartilagenous stage (chondroblasts) which are then replaced by bone (endochondral bone formation).

detailed process: mesenchymal stem cells give rise to cartilagenous precursors for your bones, these precursors roughly same shape/proportion as our bones, they dont have as much detail. The center the cartilaginous precursor= primary ossification center, where blood vessels invade into. The cartillage starts to hypotrophy and die, so cartillage in center is deteriorating, blood vessels invade in and with the blood vessel enter the bone cells. cartillage deteriorates and is REPLACED WITH BONE it doesnt turn into bone. Now we have secondary ossification centers which do the same development, until all that is left is a thin line of hyaline cartillage between the primary + secondary ossification– this is called epiphyseal GROWTH PLATE.. reason we are left with the cartillage is because this produces our longitudinal growth postnatally.

Question 4

Describe the epiphyseal growth plate + how it contributes to postnatal bone growth

Answer 4

At birth bones are not fully ossified, throughout childhood + puberty u have gaps in the bones .. Growth plates are open, these plates fuse towards end of puberty .. 18-25. Once they’ve fused u can no longer grow.
One end of growth plate is cartillage the other end is bone.
So towards cartilagenous end u have resting zone, then the chondrocytes stack on eachother (CALLED PROLIFERATION ZONE.. As they proliferate they increase the length of your epiphyseal growth plate and therefore the length of ur bone) they then hypotrophy (grow big) then after hypotrophying they die and leave the scaffold that bone cells (through blood) move into and calcify. This is how the bone is getting longer longitudinally.

what are the empty spaces seen in a growth plate histological pic?
dead chondrocyte space left behind

Question 5

Describe appositional bone growth (bone getting wider)

Answer 5

happens throughout whole life, it’s a form of Intramembranous ossification.
On outside of bone, periosteum, which cintains osteoprogenitor cells. The osteoprogenitors differentiate into osteoblasts. These osteoblasts secrete new bone matrix doing so in a ridged manner around blood vessels until they completely inclose the blood vessel in bone. To create a new central canal.
As new bone is added onto outside, bone must be removed from inner medullary cavity, this maintains the thickness of the cortical bone.

Question 6

what is the difference between lamellar + woven bone

Answer 6

Two types of bone can be identified according to the pattern of collagen forming the osteoid: Woven bone: characterized by a haphazard organization of collagen fibres and is mechanically weak. Lamellar bone: characterized by a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong.

Vast majority of bone in body is lamellar, woven bone is laid down v quickly so u find it in pathological situations (or IN childrens (immature) bone, after fracture during healing)
Lamella = means layered

Question 7

The outer layer of a bone is called compact or _______ bone. This layer is hard and particularly solid, and it makes sure that our bones can withstand daily physical strains. The outer layer has a thin coating called the ________

Answer 7

The outer layer of a bone is called compact or CORTICAL bone. This layer is hard and particularly solid, and it makes sure that our bones can withstand daily physical strains. The outer layer has a thin coating called the PERIOSTEUM

Question 8

what is a basic microscopic unit of bone

Answer 8

The basic microscopic unit of bone is an OSTEON (or Haversian system). Osteons are roughly cylindrical structures that can measure several millimeters long and around 0.2 mm in diameter. Each osteon consists of a lamellae of compact bone tissue that surround a central canal where u find an artery a vein + nerve aka neurovasculature (Haversian canal)

Osteons are the functional units of cortical bone. Osteons can be divided into primary osteons and secondary osteons. Primary osteons are adjacent to the primary bone where the blood vessels are surrounded by few circular lamellae.

Question 9

Name the 4 types of bone cell and where are they derived from

Answer 9

1) osteoprogenitor cell
2) osteoblast
3) osteocyte
4) osteoclast
Osteoclasts are derived from monocytes whereas osteoblasts, osteocytes are derived from osteoprogenitor cells

Osteoprogenitor cells differentiate from mesenchymal stem cells in bone marrow. Bone marrow can, therefore, be seen as a reservoir of osteoprogenitor cells, which differentiate further into pre-osteoblasts, which then develop into mature osteoblasts on reaching the substrate onto which bone will be deposited.

Question 10

what is the function of an osteoprogenitor cell?

Answer 10

Mesenchymal cells which maintain the cell population in bone.

Differentiate into osteoblasts.

Located in the medullary cavity and periosteum of bone.

Question 11

what is the function of an osteocyte?

Answer 11

Cyte= cell

Embedded within the osteons of bone

Maintain the bone in response to different demands

Question 12

what is the function of an osteoblast

Answer 12

Blast= precursor

Create new bone (osteoid) aka function: bone formation

When surrounded by osteoid become osteocytes

Question 13

what is the function of an osteoclast

Answer 13

Clast= to break

Derived from a different cell progeny, derived from monocytes

Break down bone to release calcium and remove damage (aka BONE RESORPTION)

Question 14

what bone of the body develops by both intramembranous + endochondral ossification

Answer 14

the clavicle develops by both intramembranous + endochondral ossification

clavicle is also the first bone to start ossification and last bone to fully ossify

Question 15

when does bone ossification start?

Answer 15

Bone ossification, or osteogenesis, is the process of bone formation. This process begins between the sixth and seventh weeks of embryonic development and continues until about age twenty-five; although this varies slightly based on the individual

Question 16

postnatal longitudinal growth is achieved by ______ ______. Postnatal appositional growth (widening) is achieved by ________ _______ (this continues for the rest of your life, no limit to when it stops)

Answer 16

postnatal longitudinal growth is achieved by ENDOCHONDRAL OSSIFICATION. Postnatal appositional growth (widening) is achieved by INTRAMEMBRANOUS OSSIFICATION (this continues for the rest of your life, no limit to when it stops)

Question 17

where are osteoprogenitor cells made

Answer 17

Osteoprogenitor cells differentiate from mesenchymal stem cells in BONE MARROW.

Bone marrow can, therefore, be seen as a reservoir of osteoprogenitor cells, which differentiate further into pre-osteoblasts, which then develop into mature osteoblasts on reaching the substrate onto which bone will be deposited

Question 18

what is an osteoid

Answer 18

Osteoid is an unmineralized organic tissue that eventually undergoes calcification and is deposited as lamellae or layers in the bone matrix.

Osteoblasts begin the process of forming bone tissue by secreting the osteoid as several specific proteins.

Osteoid consists mostly of a fibrous protein called collagen, while the mineral complexes are made up of crystals of calcium and phosphate, known as hydroxyapatite, that are embedded in the osteoid. Bone also contains nutritive cells called osteocytes.

Question 19

what is the difference between cortical vs trabecular bone

Answer 19

Cortical bone is the hard, dense bone that makes up the outer layer of all bones. It’s strong and tough, and it’s what gives bones their smooth, white, and solid appearance.

Trabecular bone, also known as cancellous or spongy bone, is found inside bones and is more porous and less dense than cortical bone. It looks like a honeycomb, with a network of spaces that make it lighter and capable of absorbing impacts.

In simple terms, cortical bone is like the hard exterior of a chocolate bar, while trabecular bone is like the airy, bubbly chocolate you might find inside. Cortical bone gives strength for support and protection, while trabecular bone helps to absorb shock and is where bone marrow is found.

Cortical bone is dense and solid and surrounds the marrow space, whereas trabecular bone is composed of a honeycomb-like network of trabecular plates and rods interspersed in the bone marrow compartment. Both cortical and trabecular bone are composed of osteons. Cortical osteons are called Haversian systems.